Devices have been used to harness wind energy for hundreds of years. In more recent times, the search for alternative sources of energy has generated renewed interest in wind energy, in an effort to replace or at least supplement coal, gas, nuclear and other energy sources. Many wind turbine designs are known, but these devices have so far been unable to compete economically with more conventional energy sources. The known devices generally require the expensive construction of heavy blades supported on huge towers. When built on a large scale, the known devices using propeller type blades encounter serious problems due to their weight and cantilevered design. The blades are known to crack due to fatigue failures, or even be thrown from the turbine, resulting in high safety and maintenance costs and reduced reliability. Because of their size and weight, vibrations during use, and the risk of a thrown blade, the known devices cannot be used on top of buildings.
A tilted windmill is disclosed in U.S. Pat. No. 4,242,043. Instead of a large supporting tower, a horizontal shaft is supported by a simple staying device. The windmill is mounted on a turntable, so that the propeller and shaft assembly can be rotated in order to follow the wind.
In some wind turbines the rotor can be driven by a sail in place of propellers. For example, the Cretin water pump, designed hundreds of years ago, uses a jib-type sail mounted on a mast that terminates in a hub and is braced by guy wires and a forward facing spar. In some modern designs, a shock cord o the tack of the sail can be used to allow it to luff when the wind is too strong.
Sailwing designs are also known. A sailwing is a double-sided sail having an airfoil shape that acts like a wing rather than an ordinary sail. For example, the Princeton sail wing uses many rigid members, and has an aluminum sheet on its leading edge. The known designs use masts to support the sails, are cantilevered, and sometimes rely on supporting guy wires.
Some turbines employ a rotor having a spoke-like structure, like a bicycle wheel. For example, the American Wind Turbine uses a spoked rotor to support rigid blades and a solid rim, and the rim is further supported by rigid spars. The Unified Wind Dynamo also uses a spoked rotor.
A multiple wind turbine tethered airfoil wind energy conversion system is disclosed in U.S. Pat. No. 4,285,481, which provides a means for supporting a plurality of wind turbines aloft on the same tethered airfoil. U.S. Pat. No. 4,166,596 discloses an airship power turbine, where power transmission cables are used to connect airborne turbine wheels carried aloft by a tubular-shaped aircraft with generator drive shafts below.
All of the known devices suffer from disadvantages, both in cost and efficiency, and are substantially different in structure and function from the invention claimed herein.